A gas proportional counter is an x-ray detector used principally for the measurement of radiation of light elements (wave-lengths greater than 2A).
Such a counter consists of a metal cylindrical chamber closed at both ends and a very thin metal wire, usually tungsten, positioned along the axis of the cylinder to which a high potential, e.g. 1500-2000v is applied. The cylindrical wall is grounded so that the wall constitutes a cathode and the central wire an anode.
An aperture in the wall, sealed with an aluminized plastic wall (e.g. Mylar) a registered trademark of E. I. DuPont de Nemours for a polyethylene terephthalate resin serves as a window (with a minimum of absorption of incoming radiated).
The counter is filled with an ionizable gas, generally argon-methane. To compensate for the effects of loss of gas through the window a flow of gas through the counter may be provided.
X-ray quanta passing through the window ionize the gas which consists of producing pairs of ions (electrons + positive ions). Very little energy is required to ionize the gas, usually of a few electron volts depending on the nature of the gas used. This energy, called "ionization potential" is 26.4 eV for argon. The maximum number of pairs of ions formed is given by the expression: ##EQU1## WHERE N = NUMBER OF ION PAIRS, E = radiation energy is electron volts, and e = effective ionization potential. The electrical field originating from the difference in potential between the electrode of the counter makes it possible to separate these ion pairs.
The field causes electrons to move towards the anode (central wire) and the positive ions towards the cathode. The primary electrons may collide with other gas atoms and can ionize them. If the kinetic energy of the electron is large enough, a new electron, in addition to the primary electron, will be released after the collision and will in turn collide with gas atoms and release new electrons. This process can continue producing gas amplification.
The region where this ionization takes place in an avalanche is defined by a critical field strength value for which the acceleration of electrons between two collisions is sufficient to cause ionization. As the electrical field strength inside the counter is dependent on the inverse of the logarithm of the ratio of the counter-and the anode diameters, the field strength decreases very quickly with distance from the anode and the region in which ionization avalanches occur is very close to the anode.
As electrons reach the central wire, or anode, a current pulse is produced which can be in an output circuit. Each such pulse corresponds then to an ionization event and the efficiency of the counter is a measure of the number of counts it can produce in response to ionizing radiation.
In addition, the pulse height or size depends on the primary ionization and this makes possible the discrimination between radiation types which differ in the primary ionization that is produced. Thus, the resolving power of the counter can be expressed in terms of the ratio in % of the width (b) at half maximum to the average pulse height (a), or: ##EQU2##
Because of statistical fluctuations, particularly in the primary ionization, the size of the pulses measured for one element is not constant. However, the statistical fluctuations follow the Gaussian law which makes it possible to define the theoretical resolution R.sub.Th of the counter and this can be shown to be: EQU R.sub.Th = .sqroot.4.5.lambda.e where .lambda. is the wave-length of the ionizing radiation and e is the effective ionization potential of the gas.
Positive ions, because of their mass, move slowly toward the grounded cathode. As a result, a positive ion sheath is formed which alters the electrical field between the anode wire and cathode. Because of this condition, the functioning of the detector is affected in various ways. The most serious are an increase in detector dead time and a shift in the pulse amplitude.
The dead time refers to the intervals during which the detector is not active. The measured count rate, therefore, will be lower than the true count rate.
The shift in pulse amplitude affects the application of pulse height selection since pulses will move progressively outside the window setting of a pulse height analyzer, which is conventionally used to select pulses of given amplitude, as the counting rate increases. The result is a serious decrease in the true rate.
It is a principal object of the invention to provide a counter for detecting ionizing radiation having an improved resolution approaching the theoretical value.
It is a further object of the invention to provide a gas counter for detecting ionizing radiation having reduced dead time.
It is a still further object of the invention to provide a gas counter for detecting ionizing radiation which produces pulses of suitable amplitude for proportional counting.
It is yet another object of the invention to provide an improved gas counter for detecting ionizing radiation in which the positive ion sheath is utilized to control the operation of the counter more effectively, and to control a source of ionizing radiation.